The present invention relates to a pigment coated phosphor used in manufacturing the phosphor layer of a high contrast cathode ray tube for consumer use and to a method for manufacturing the same, and particularly to a pigment coated phosphor in which the dispersibility is good and flocculation between phosphor particles is largely prevented
Generally, adhesion of red, green and blue pigment particles on the surface of respective red, green and blue emitting phosphors employed when manufacturing a phosphor layer of a cathode ray tube, enhances color purity of the emitted light from the phosphor by a so-called filter effect, absorbing or reducing light of an undesired wavelength and largely enhancing the contrast of the reproduced image by absorbing the external light reflected on the panel surface.
These kinds of pigment coated phosphors are generally manufactured by coating pigment particles on the surface of phosphor particles through preparing phosphor dispersion and pigment dispersion, mixing them with a binder and then hardening the binder using a hardening agent.
Many methods for manufacturing such pigment coated phosphors are well known. For example, in Japanese Laid-open Patent sho 50-56146 and 51-80296, methods for manufacturing pigment coated phosphors employing gum arabic and gelatin as binders are disclosed. Also in U.S. Pat. No. 4,049,842, a method for manufacturing a pigmented phosphor using acryl emulsion is disclosed.
The pigment coated phosphor manufactured by the above-mentioned methods should satisfy the following conditions so as to provide a phosphor having good characteristics.
First, pigment particles should adhere to the surface of the phosphor with sufficient strength so that the pigment particles may not detach from the bare phosphor during the slurry combining process; second, the pigment particles should be uniformly distributed on the surface of the phosphor during the pigment coating process and the pigment coated phosphors should not flocculate each other; third, the employed binders should be cleared through evaporation during a baking process when manufacturing the cathode ray tube and; fourth, an adequate amount of pigment particles should be coated on the surface of the phosphor, considering the luminance deterioration and reflection ratio of the phosphor, etc.
According to the conventional method, the above-required various conditions are almost satisfied for the common cathode ray tube. However, cathode ray tubes of high contrast and high definition, following the trend of recent times, require far more reduction of luminance deterioration, restraint of flocculation and consistent adhesion of the pigments.
When using an acryl emulsion as a binder, high flocculation between phosphor particles occurs and this gives an undesirable phosphor having defective characteristics and results in a mesh-plugging phenomena. When using gelatin/gum arabic as the binder, though the ratio of flocculation occurrence is lower than that occurring when employing acryl emulsion, the adhesion power of the pigment particles to the surface of the phosphor is too weak. Moreover, when employing gelatin, since the change of the quality of the gelatin results in the pigment's detachment from the phosphor, another method for manufacturing a pigment coated phosphor employing an agent which can inhibit the decomposition of gelatin is disclosed (Japanese Patent Publication No. sho 61-1478).